1. Field of the Invention
The present invention relates to a digital circuit for a passive radio frequency identification (RFID) tag and, more particularly, to a low power digital circuit for a passive RFID tag, which is divided into sequential blocks according to the flow of data and transmits an optimized clock signal to each of the blocks only when the block should perform an operation.
2. Discussion of Related Art
In a passive RFID system, when a reader produces a radio frequency (RF) signal, a tag converts the RF signal into direct current (DC) power and employs the DC power as power required for driving the entire circuits therein. Thus, the tag needs to be designed such that it operates at low power. Such a tag includes an analog circuit, a digital circuit, and a memory. The analog circuit includes a driving power generation block and a clock signal generation block. The digital circuit analyzes a command issued from the reader and supports an RFID protocol. Because the digital circuit consumes more power than the analog circuit or the memory, designing the digital circuit to operate at low power is necessary.
In order to embody a low power digital circuit, an application specific integrated circuit (ASIC) is more appropriate than a central processing unit (CPU) or a digital signal processor (DSP). In the case of a CMOS ASIC, power is mostly consumed due to a current that flows when a capacitor of a chip is charged or discharged. This can be approximated as shown in the following Equation:Consumed power=Vdd2*C*Freq,                where Vdd refers to a voltage, C refers to a capacitance of a logic, and F refers to a speed at which a capacitor is charged/discharged.        
In this connection, the conventional digital circuit for the passive RFID tag, which is synchronized with a clock signal, consumes different amounts of power according to a clock rate. Therefore, in order to embody a low power clock-synchronized digital circuit, the digital circuit should be operated at such a minimum clock rate that meets required performance.